1. Technical Field
The present invention relates to a display apparatus including a liquid crystal panel which displays an image and a plurality of light sources which irradiate the liquid crystal panel with light, and a television receiver.
2. Description of Related Art
When a liquid crystal display (LCD) displays, for example, an image having movement, an afterimage phenomenon, in which an outline of a moving portion is obscurely perceived in a human vision, may easily occur. The reason is that the liquid crystal display is a hold type display apparatus which holds display light for one frame period after writing of data in a pixel is ended and before the next writing of data in the pixel is started.
Herein, as a technique for improving moving image display performance of the liquid crystal display, a black insertion method has been known in the art. The black insertion method is a technique in which an image signal and a black signal are written in the liquid crystal display within each frame period of an image frame, thereby the black signal is inserted between the image signals which follow in time sequence, so as to perform a pseudo impulse type display control. In addition, a backlight scanning technique, in which a backlight (light source) is divided into a plurality of regions and the backlight is turned on for each region, has been known in the art. The backlight scanning technique is a technique in which a turning off period of the backlight is set within a period of displaying one image frame by the liquid crystal display, thereby a display period of one image (image frame) is shortened, so as to perform the pseudo impulse type display control. By these techniques, it is possible to reduce the afterimage phenomenon occurring at the time of displaying the moving images in the liquid crystal display.
Hereinafter, processing of controlling the liquid crystal panel and the backlight simultaneously using the black insertion method and the backlight scanning technique will be described.
FIG. 1 is a schematic view illustrating a relationship between the liquid crystal panel and the light source in the liquid crystal display. FIG. 1 illustrates a liquid crystal panel 90 as seen from a display surface side. The liquid crystal display illustrated in FIG. 1 is a light guide type (edge light type) liquid crystal display, and includes the liquid crystal panel 90, and light sources 91 to 96 which irradiate the liquid crystal panel 90 with light. In addition, the liquid crystal display includes a light guide plate (not illustrated) which is disposed on a back surface side (surface side opposite to the display surface) of the liquid crystal panel 90 to guide light from the light sources 91 to 96 to the liquid crystal panel 90. The liquid crystal panel 90 is configured to perform the display control of the image for each display region (first region, second region, . . . , and sixth region) which is divided into six, for example, in a longitudinal direction (vertical direction) and respectively has a strip shape. The light guide plate has a size substantially the same as the liquid crystal panel 90. The light sources 91 to 96 are provided at positions respectively facing both end surfaces of the light guide plate in a lengthwise direction (lateral direction) thereof, that is, positions respectively corresponding to two short sides of each display region of the liquid crystal panel 90. Thereby, the respective light sources 91 to 96 irradiate each display region of the liquid crystal panel 90 with light through the light guide plate.
FIG. 2 is a view for describing control processing for the liquid crystal panel 90 and the light sources 91 to 96. FIG. 2 illustrates a change in the display image and a change in values of current applied to the light sources 91 to 96 with the passage of time, in each display region (first region, second region, . . . , sixth region) of the liquid crystal panel 90. Further, FIG. 2 illustrates a time axis (lateral axis) for showing the change in the display image, and a lateral axis (time axis) and a vertical axis (current value) for showing the change in the values of current applied to the light source 91, only for the first region, and the axes are not illustrated for the other display regions.
In an example illustrated in FIG. 2, displaying of the image signals in each display region is sequentially started with predetermined time-lags in an order of the first region, the second region, . . . , and the sixth region, and one frame (image frame) is displayed on the liquid crystal panel 90 as a whole. In addition, the black signals (black images) are inserted into each frame to be displayed on the liquid crystal panel 90, and in the image signals displayed in each display region, the black signals are respectively inserted between the image signals which follow in time sequence. The respective light sources 91 to 96 are controlled so as to irradiate each display region with light during a latter period within a period in which each display region displays each image signal. Specifically, the light sources 91 to 96 are turned on when a predetermined time t has elapsed after the displaying of the image signal in the corresponding display regions is started, and are turned off when the displaying of the next black signal in each display region is started. Further, the values of current applied to the respective light sources 91 to 96 are the same as each other and the respective light sources 91 to 96 irradiate the display regions with light having the same luminance as each other.
In this way, moving images can be sharply displayed and the moving image display performance of the liquid crystal display can be improved, by simultaneously using the black insertion method and the backlight scanning technique.
However, when performing the control processing as illustrated in FIG. 2, the light from the light sources 91 to 96 which have been turned on enters (is leaked) into the display region corresponding to the light sources 91 to 96 which have been turned off, thereby the luminance distribution is not uniform in the liquid crystal panel 90. For example, in a period in which a first image signal is displayed in the first region, the light source 92 as well as the light source 91 corresponding to the first region is turned on, and the light from the light source 92 also enters into the first region. In addition, in a period in which the first image signal is displayed in the second region, the light sources 91 and 93 as well as the light source 92 corresponding to the second region are turned on, and the light from the light sources 91 and 93 also enter into the second region. Similarly, in a period in which the first image signal is displayed in the sixth region, all the light sources 91 to 96 are turned on, and the light from all the light sources 91 to 96 enter into the sixth region. In a period in which the black signals are displayed in each display region, even when the light from the light sources 91 to 96 other than the light sources 91 to 96 corresponding to each display region enter into each display region, an effect by the entering light is small. In this way, when the image signals are displayed in each display region, an amount of light entering into each display region is different, and therefore a deviation in luminance distribution occurs in the liquid crystal panel 90. As illustrated in FIG. 2, when displaying the image on the liquid crystal panel 90 by scanning each display region in the order of the first region, the second region, . . . , and the sixth region, the luminance at a lower portion (sixth region) of the screen is higher than the luminance at an upper portion (first region) of the screen.
FIGS. 3A and 3B are views for describing the deviation in the luminance distribution in the liquid crystal panel 90. FIGS. 3A and 3B illustrate a change in luminance of the image signals and a change in luminance in the display screen with the passage of time, in the first region and the sixth region. As illustrated in FIGS. 3A and 3B, in the first region and the sixth region, the luminance based on the first image signal is substantially the same as each other. Meanwhile, a difference in the luminance on the display screen of the first image signal, that is, in the luminance after the light from the light sources 91 to 96 passes through the first region and the sixth region, which are displaying the first image signal, occurs between the first region and the sixth region. The reason is that the first region is in the display state of the black signal during a part or all of the periods in which the light sources 92 to 96 are turned on, and therefore the effect by the leakage of the light from the light sources 92 to 96 to the first region is small, while the sixth region is in the display state of the image signal for all the periods in which the light sources 91 to 95 are turned on, and therefore the effect by the leakage of the light from the light sources 91 to 95 to the sixth region is large. When the image signals are displayed in each display region in an order from the upper portion of the screen to the lower portion thereof, the above effect is increased toward the lower portion of the screen. Thereby, when the image signals having substantially the same luminance as each other are displayed in each display region, the luminance is increased (is higher) toward the lower portion of the screen.
In order to suppress the above-described deviation in the luminance distribution in the liquid crystal panel 90, it is conceivable that an appropriate pattern is formed on the light guide plate, so that each light from the light sources 91 to 96 does not leak to a region other than the display regions corresponding thereto.
In addition, Japanese Patent Application Laid-Open No. 2010-276928 proposes a configuration in which a light source corresponding to a lower region of a liquid crystal panel is turned off while the light source corresponding to an upper region thereof is turned on, so that the turn on time (lighting time) of the light sources corresponding to the respective regions of the liquid crystal panel does not overlap with each other. Thus, in the configuration in which a deviation in the luminance distribution in the liquid crystal panel occurs, the light source corresponding to the lower region of the liquid crystal panel is turned on while the light source corresponding to the upper region is turned off, and a ratio of the lighting time of the light source corresponding to the lower region is more increased than the ratio of the lighting time of the light source corresponding to the upper region and a center region, and thereby suppressing the deviation in the luminance distribution in the liquid crystal panel. By this, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-276928, even when the light source corresponding to the lower region is turned off so that the lighting time of the light sources corresponding to the respective regions of the liquid crystal panel does not overlap, a decrease in the luminance in the lower region can be suppressed, and thereby suppressing the deviation in the luminance distribution in the liquid crystal panel.